FIG. 10 shows a solar cell element disclosed in Patent Literature 1. This solar cell element comprises a glass substrate 91, a first transparent conductive film 92, a metallic thin film 98, a p-type layer 95, an I-type layer 96, an n-type layer 97 and a second transparent conductive film 94 in this order. The metallic thin film 98 has a high work function. The incident light travels to the layers 95-97 through the glass substrate 91. Examples of the material of the metallic thin film 98 include rhodium, iridium, gold, palladium, and nickel. The metallic thin film 98 has a thickness of 1-10 nanometers.
FIG. 11 shows an organic solar cell element disclosed in Patent Literature 2. This solar cell element comprises a substrate 920, a first electrode 932, a first metal layer 942, an active layer 950, a second metal layer 944, and a second electrode 934 in this order. Examples of the materials of the first electrode 932 and the second electrode 934 include indium oxide, zinc oxide, tin oxide, and a mixture thereof. The first metal layer 942 has a work function of not less than 3.7 eV and not more than 5.5 eV. An oxide of the first metal layer 942 has a semiconductor property. The first metal layer 942 has a thickness of 2 nanometers-50 nanometers. Examples of the material of the first metal layer 942 include zinc, tin, titanium, and niobium. The second metal layer 944 is the same as the first metal layer 942.
Patent Literature 3, Patent Literature 4, and Patent Literature 5 disclose that vanadium, copper, zinc, gold, chrome, cobalt, nickel, iron, tungsten, silver, tin, titanium, palladium, platinum and alloys thereof have a high work function.